Secondary batteries which have been generally used are mainly classified into two categories, namely, the categories of "lead battery" and "nickel-cadmium battery". In particular, the nickel-cadmium battery has been used widely because of its high-rate discharge characteristics, its long lifetime and so on, compared with the lead battery. On the other hand, development of a secondary battery capable of being charged in a short time and development of a high-capacity secondary battery have been demanded with the reduction of size and weight of electronic appliances in recent years.
A sealed nickel-cadmium battery has several problems with respect to the reduction of the charging period thereof and improvement in the capacity thereof.
The problem with respect to the reduction of the charging period is as follows.
It is known that the sealed nickel-cadmium battery can be sealed by so-called reserved cadmium hydroxide contained in negative electrode in an amount of electricity more than that of nickel hydroxide contained in the positive electrode. In an overcharge region oxygen gas is generated from the positive electrode before hydrogen gas is generated from the negative electrode. The oxygen gas is absorbed into the negative electrode by the reaction expressed by the following formula (1). EQU O.sub.2 +H.sub.2 O+Cd.fwdarw.Cd(OH).sub.2 ( 1)
As a result, the pressure of gas in the battery increases as the partial pressure of oxygen increases. If the pressure of gas is less than the pressure by which a safety valve of the battery is operated, there is no generation of hydrogen gas from the negative electrode, but there is generation of heat.
This means that the closed characteristic of the battery cannot be maintained if the magnitude of a charging current in the overcharge period is not more than a value corresponding to the oxygen gas absorbing rate into the negative electrode.
The method for charging the battery is mainly classified into two types, namely, a constant-current method, and a constant-voltage method, which is simpler in charging control than the former.
In general, the sealed nickel-cadmium battery is charged with a constant current, because the constant-voltage charging method used as a general method for the lead battery cannot be easily used for the sealed nickel-cadmium battery. The reasons are that the increase of the voltage of the sealed nickel-cadmium battery in the final charge stage is only from about 100 to about 150 mV, and that the increase of the voltage becomes smaller when the temperature is high or when the active material in the positive electrode is crystallized by aging phenomenon.
Therefore, the sealed nickel-cadmium battery must be charged with constant-current. And the value of charge current must be smaller than a value corresponding to the oxygen absorbing rate into the negative electrode. So that, the absorption of oxygen gas by the negative electrode must be improved in order to charge the sealed nickel-cadmium battery more rapidly. Because the oxygen gas absorbing rate into the negative electrode is proportional to the partial pressure of oxygen gas, the battery suitable for rapid charging is typically a cylindrical-form battery rather than a square-form battery. In the cylindrical-form battery, the charge is, however, limited to about 1 CA.
If the case of hydrogen gas is generated from the negative electrode in a charging period, the following disadvantage arises. A small part of the hydrogen gas can be absorbed with the positive electrode, but a large part of the hydrogen gas remains in the battery, so that the partial pressure of oxygen gas decreases as a hydrogen gas is accumulated in the battery. When the internal pressure of the battery reaches the pressure at which the safety valve can be operated, the gas in the battery flows out. This means that the quantity of electrolyte is reduced, thereby lowering the capacity of the battery.
The problem with respect to improvement in energy density of the sealed nickel-cadmium battery is as follows.
The cause of prevention of the high capacity of the sealed nickel-cadmium battery is mainly in the cadmium electrode thereof. The cadmium electrode includes reserved cadmium hydroxide, non-chargeable cadmium hydroxide, and pre-charge metallic cadmium, as except the active material related to the charge/discharge reaction with the positive active material. In particular, the main cause is the reserved cadmium hydroxide.
As described above, the reserved cadmium hydroxide serves to maintain sealed condition of the battery. The quantity of the reserved cadmium hydroxide must be established to be more than a value for compensating the quantity of oxygen spent in oxidation of a nickel substrate in the positive electrode, the quantity of oxygen spent in oxidation of a separator, and the quantity of oxygen accumulated in the battery in an overcharge situation. In general, the theoretical capacity of the reserved cadmium hydroxide ranges from 40 to 100% of the theoretical capacity of the positive active material electrode, though it varies according to the quality of the separator, the form of the battery and the condition of use of the battery. The reserved cadmium hydroxide which does not contribute to the capacity of the battery is unwelcome from the point of view of the energy density of the battery, but is necessary for keeping the sealed condition of the battery.
Recently, a proposal has been made to solve the aforementioned problem in the nickel-cadmium battery (Japanese Patent Application No. 62-86582) The proposal relates to a battery having a cadmium negative electrode that have large hydrogen over-potential and includes little reserved cadmium hydroxide. According to the proposal, rapid charge without temperature compensation can be carried out by controlling the charge current with detecting the voltage change of battery in the final stage of the charging period. The voltage change of the battery in the final stage of the charging period is caused by the potential change to the hydrogen evolution of negative electrode. However, this proposal has a disadvantage in that the discharge capacity of the battery is lowered in repetition of charge/discharge cycles, in particular, at a low temperature because the charging efficiency of the negative electrode is not so great.
After the above proposal, another proposal has been made to improve the charging efficiency of the negative electrode by adding nickel hydroxide or nickel oxide (Japanese Patent Application No. 63-13345.) According to this proposal, the change in the discharge capacity during the charge/discharge cycles of the battery can be improved remarkably compared with the prior proposal. However, the battery has two disadvantages, as follows. The first disadvantage is in that the hydrogen over-potential of the negative electrode is lowered by addition of nickel hydroxide. This is undesirable in that when the battery is charged with a constant voltage, the range of the allowed voltage becomes narrow. The second disadvantage is in that the theoretical capacity density of the cadmium negative electrode is lowered by addition of nickel hydroxide.
One of the methods for improving the charging efficiency without reduction of the hydrogen over-potential of cadmium electrode comprises preparing cadmium hydroxide contained in the cadmium negative electrode in the form of .gamma.-Cd(OH).sub.2. In general, cadmium hydroxide contained in the cadmium negative electrode is .beta.-Cd(OH).sub.2. As reported in the 15th International Power Source Symposium, P1 (1986), in the Journal of Electrochemical Society, 117, 583 (1970), in the Journal of Applied Chemistry U.S.S.R., 50, 2251 (1976) and in the Journal of Applied Chemistry U.S.S.R., 51, 1846 (1978), it is known that .gamma.-Cd(OH).sub.2 is produced temporarily under certain conditions, viz., only in the case where the atmospheric temperature is low, or in the case where an electrolytic solution containing sodium hydroxide is used.
However, no process for producing .gamma.-Cd(OH).sub.2 stably, easily, and quantitatively has been established. Therefore, performance of a battery using .gamma.-Cd(OH).sub.2 as negative active material has been unknown.